Implantable electronic devices are in use providing electronic pulses to stimulate tissue via a lead extending from an implanted pulse generator to a desired internal location. An example of this type of technology is a pacemaker and a pacing lead which provides electrical stimulation to the heart. The pacemaker is usually implanted in a subcutaneous cavity, and the leads extend either transvenously to the internal cavities of the heart, or to patch electrodes located on external surface of the heart.
The leads generally include at least one electrode located at a distal end and an electrical connector for interconnection to the pulse generator at the proximal end. The connector at the proximal end and the distal electrode are interconnected by at least one conductor extending through an insulated body. It is common for the leads to include two or more electrodes and two or more electrical contacts at the connector.
The connector is inserted into a receiving orifice in a header portion of the pulse generator. The header portion of the pulse generator defining the receiving orifice may be formed from an epoxy material which is assembled and bonded to the main body of the pulse generator. The main body of the pulse generator is generally a metallic self-contained housing or can, which encloses the source of electrical energy and electrical circuitry for controlling the electrical stimulus delivered by the lead.
An alternative to the epoxy header design is discussed in detail in Truex et al. U.S. Pat. No. 4,934,366, which also teaches locking mechanism for securing the lead. The Truex et al. patent is assigned to the assignee of the present invention and is herein incorporated by reference.
Electrical contact elements, such as toroidal springs, are mounted in the header to make electrical contact with the electrical contacts of the lead connector, and feed the pulses to the body from the pulse generator. Generally, the electrical elements in the header are passive, in that they rely on deformation to maintain contact with the electrical contacts of the connector.
In the design of the lead connector and the pulse generator, it is important for the lead to be safely secured to the pulse generator to prevent inadvertent decoupling. In addition fluids must be prevented from invading the connection and contacting the electrical contacts. Generally, passive seals such as O-rings or circumferential ridges on the connector have been relied upon to passively prevent the invasion of fluids. However, these types of passive seals are quite delicate and tend to take a "set" over time and can possibly permit the invasion of body fluids.
It is also required that the insertion force for the connector be maintained below a fixed standard. Preferably, the force required to insert the connector into the pulse generator should be minimized. There are ISO industry standards for the maximum allowable insertion force of leads. The cumulative forces of the lead connector engaging passive elastic seals and spring-type contacts on the header connector can exceed these force limits.
It is also required that the interconnection between the pulse generator and the lead be defeasible, because the pulse generator must be capable of being removed while the lead will remain in place to be used with a new pulse generator. Accordingly, a mechanism for securing the connector in the pulse generator is required which will allow the lead connector to be removed from the pulse generator after many years without damage to the lead.
Various techniques have been used to actively secure the connectors to the pulse generator, for example, by the use of a set screw. Frey et al. U.S. Pat. No. 4,860,750 details a lead and pulse generator having a lead locking mechanism which includes a wedge member which interengages the lead to deform the lead and prevent disconnection. The wedge is inserted into a generally tangential slot in the header, and forces the lead to be deformed off center during insertion of the wedge. The lead may remain slightly compressed due to the shape of the wedge in a design according to Frey et al.
While Frey et al. is successful in securing the connector to the pulse generator, it has the undesirable effects of displacing the passive seals or O-rings, and also deforming the lead body. When the locking mechanism of Frey et al. is inserted, the lead body is displaced from axial alignment in the receiving orifice. This displacement causes the passive seals encircling the lead body to be compressed on one side, and potentially disengage or lose some of their delicate, passive loading from the opposite side, thereby opening a path for fluids to leak past the passive seal. In addition, most leads contain a helical wound conductor. Deformation of the lead body caused by the locking mechanism may result in crushing or flattening of the helical conductor, possibly leading to breakage.
Accordingly, it would be beneficial to have a pulse generator and lead assembly including an easily defeasible retention device which includes an alternative to the passive seals of the prior art, which has a minimum insertion force, and which secures the lead connector without damaging the conductors therein.